Field of the Invention
The disclosure relates generally to a layout placement of a current source device, and more particularly it relates to a layout placement of a current source adapted for a current-steering digital-to-analog converter.
Description of the Related Art
Arrays of equal value current sources are used to define the most significant bits in high-speed digital to analog converters (DACs). Accurate matching between these current sources, typically implemented with MOS transistors, is essential to achieve a DAC that is highly linear without being trimmed or calibrated.
The individual current sources of a DAC are mismatched for a number of reasons. Random fluctuations in threshold voltage and charge carrier mobility cause a random mismatch between the current sources. Gradients in oxide thickness, mechanical stress and resistive voltage drops in supply lines cause linear, parabolic and higher order gradients across the current source array, resulting in a nonlinear DAC transfer function.
Therefore, the layout of thermometer coded most significant bits (MSBs) for a current-steering DAC must be done with utmost care to avoid systematic errors due to linear and higher order process gradients.